cluster: avoid race enabling debugger in worker

Previously if a worker's state machine had already transitioned into the
'listening' state when it received the message enabling the debugger,
the worker would never enable its debugger.

Change the logic to allow the 'listening' as a valid state for enabling
the debugger.

Fixes #6440

Signed-off-by: Julien Gilli <julien.gilli@joyent.com>